The air-fuel mixture which is supplied to a combustion chamber, for example to the inlet manifold of an internal combustion (IC) engine, can be controlled by sensing the exhaust gases from the combustion process, and then controlling the mass ratio of fuel-air so that the output or exhaust gas will have low noxious components. Oxygen sensors can be used which provide output signals varying in value in dependence on the oxygen content in the exhaust gas, that is, whether the exhaust gas is "lean", that is, has an excess of oxygen, or "rich", that is, contains an excess of unburned hydrocarbons. Such oxygen sensors have been used in various types of control systems. On such system is described in U.S. Pat. No. 3,875,907, WESSEL et al., assigned to the assignee of this application. It has already been proposed--see German Pat. No. 24 42 229--to modify the output signal derived from an oxygen sensor by extending the output signal derived therefrom by a predetermined time duration, in accordance with a time setting. The modified output signal then permits controlling the subsequent control system to, in turn, so control the fuel-air ratio that the exhaust gases can vary between .lambda.=0.95 to 1.05, in which .lambda. is defined as the "air number" and .lambda.=1.0 refers to stoichiometric proportion of fuel and air, resulting in exhaust gases which contain neither unburned hydrocarbons nor excess oxygen. The arrangement described in the aforementioned German Pat. No. 24 42 229 does not permit operating the oxygen sensor at any desired temperature, without change of the .lambda.-value.
U.S. patent application Ser. No. 357,803, filed Mar. 12, 1982, DIETZ et al., now U.S. Pat. No. 4,419,190, Dec. 6, 1983 assigned to the assignee of this application, describes a system in which the temperature of an oxygen sensor of the type to which the present invention relates can be determined by measuring the a-c resistance thereof. A temperature signal thus can be obtained from the sensor which can be used to heat the sensor or to connect the sensor, or a control system connected thereto, only when the sensor has reached a predetermined temperature, in order to insure that the control system will, reliably, control the fuel-air ratio to the combustion process to result in optimum combustion.